The following is a summary of “Low pH condition impairs BP-IgG binding to the basement membrane zone,” published in the March 2024 issue of Dermatology by Im, et al.

Bullous pemphigoid (BP) is an autoimmune subepidermal blistering disease characterized by tense blisters and urticarial erythema. Direct immunofluorescence (DIF) detecting tissue-bound Immunoglobulin G (IgG) at the basement membrane zone (BMZ) is crucial for diagnosing BP. However, DIF sensitivity is higher for complement component 3 (C3) than IgG. The underlying mechanism for this discrepancy in sensitivity is not fully understood. For a study, researchers sought to investigate the possible reasons behind the higher sensitivity of C3 over IgG in DIF for diagnosing BP, particularly focusing on the role of pH conditions.

Ex vivo studies were conducted using sera patients suffering from BP. The initial step involved testing sera from patients showing IgG negativity by DIF to see if they reacted with the BMZ in their own DIF skin samples. Indirect immunofluorescence (IIF) was then performed with sera diluted in phosphate-buffered saline (PBS) of varying pH levels (7.4, 6.0, and 3.0). Finally, skin sections from patients with BP were pre-incubated with different pH PBS solutions before staining with anti-human IgG and C3 antibodies, and the resulting fluorescence intensities were measured.

Sera from patients who were negative for IgG by DIF still reacted with the BMZ in their skin samples. Sera diluted with pH 7.4 PBS displayed clear linear staining at the BMZ, whereas lower pH dilutions (6.0 and 3.0) resulted in reduced fluorescence intensities. Pre-incubation of skin sections with different pH PBS solutions showed that both IgG and C3 fluorescence intensities were significantly lower at pH 3.0 and 6.0 compared to pH 7.4. It suggested that low pH conditions hinder the binding of autoantibodies to the BMZ and may cause tissue-bound autoantibodies to detach.

Inflammation-induced low pH conditions impede the binding of autoantibodies to the BMZ and can cause already bound autoantibodies to detach. Complement fragments can remain at the BMZ due to activation on both IgG and cell surfaces nearby. The findings helped explain why C3 detection is more sensitive than IgG in DIF for diagnosing BP, as complement fragments are less likely to detach under low pH conditions than IgG.

Reference: onlinelibrary.wiley.com/doi/10.1111/1346-8138.17175